Electrolytic capacitor

ABSTRACT

An electrolytic capacitor includes an anode body having a dielectric layer; a solid electrolyte layer in contact with the dielectric layer of the anode body; and an electrolytic solution. The electrolytic solution contains a solvent and a solute. The solvent contains a glycol compound. The solute contains an acid component and a base component. A mass of the acid component in the solute is greater than a mass of the base component in the solute. The acid component contains a first aromatic compound having a hydroxyl group.

This application is a Continuation of U.S. patent application Ser. No.16/703,080 filed on Dec. 4, 2019, which is a Continuation of U.S. patentapplication Ser. No. 15/915,109 filed on Mar. 8, 2018, which is aContinuation of International Application No. PCT/JP2016/004271 filed onSep. 20, 2016, which in turn claims the benefit of Japanese ApplicationNo. 2015-189454 filed on Sep. 28, 2015, the disclosures of which areincorporated in their entirety by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to an electrolytic capacitor including asolid electrolyte layer and an electrolytic solution.

2. Description of the Related Art

As small-sized, large capacitance, and low equivalent series resistance(ESR) capacitors, promising candidates are electrolytic capacitorsincluding an anode body on which a dielectric layer is formed, a solidelectrolyte layer formed so as to cover at least a part of thedielectric layer, and an electrolytic solution.

For the solid electrolyte layer, a i-conjugated conductive polymer isused. From the viewpoint of improving withstand voltage characteristicsof the electrolytic capacitor, it has been proposed that a solventcontaining ethylene glycol and γ-butyrolactone is used as anelectrolytic solution (see PCT International Publication No. WO2014/021333). In addition, it has been proposed that an antioxidant isadded to an electrolytic solution for increasing a sparking voltage (seeUnexamined Japanese Patent Publication No. 2006-114540).

SUMMARY

An electrolytic capacitor according to an aspect of the presentdisclosure includes an anode body having a dielectric layer; a solidelectrolyte layer in contact with the dielectric layer of the anodebody; and an electrolytic solution. The electrolytic solution contains asolvent and a solute. The solvent contains a glycol compound. The solutecontains an acid component and a base component. A mass of the acidcomponent in the solute is greater than a mass of the base component inthe solute. The acid component contains a first aromatic compound havinga hydroxyl group.

According to the present disclosure, there can be provided anelectrolytic capacitor that is excellent in withstand voltagecharacteristics and heat resistance, and can maintain low ESR.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating an electrolyticcapacitor according to an exemplary embodiment of the presentdisclosure; and

FIG. 2 is a schematic view for explaining a configuration of a capacitorelement according to the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENT

The electrolytic capacitor is required to have low ESR, a heatresistance and so on in addition to withstand voltage characteristics.From the viewpoint of improving voltage resistance and heat resistance,it is preferable that a solute of an electrolytic solution contains anacid, and that a glycol compound is used as a solvent of theelectrolytic solution. However, when a glycol compound is used as thesolvent, ESR tends to rapidly increase earlier than expected under along-term load test conducted at 100° C. or higher although low ESR isexhibited in an initial stage.

In view of the foregoing, the present disclosure provides anelectrolytic capacitor that is excellent in withstand voltagecharacteristics and heat resistance, and can maintain low ESR.

The electrolytic capacitor according to the present disclosure includesan anode body having a dielectric layer; a solid electrolyte layer incontact with the dielectric layer; and an electrolytic solution. Theelectrolytic solution contains a solvent and a solute. The solventcontains a glycol compound. The solute contains an acid component and abase component. In this regard, however, a mass of the acid component inthe solute is greater than a mass of the base component in the solute.The acid component contains a first aromatic compound having a hydroxylgroup.

Orientation or crystallinity of a conductive polymer included in thesolid electrolyte layer can be increased by containing the glycolcompound in the solvent. An increase in the orientation and thecrystallinity of a conductive polymer improves conductivity of the solidelectrolyte layer so that ESR of the electrolytic capacitor can bereduced. Contactivity between the solid electrolyte layer and thedielectric layer is improved so that withstand voltage characteristicscan be also improved. The conductive polymer is considered to be swollenby the glycol compound. A swollen conductive polymer is likely to causerearrangement, so that orientation or crystallinity of the conductivepolymer is considered to be improved.

According to the above-described configuration, improvement of heatresistance or ripple resistance of the electrolytic capacitor can alsobe expected. This is because the glycol compound hardly volatilizes todissipate from the electrolytic capacitor to outside. It is consideredthat the electrolytic solution volatilizes to dissipate at a sealedportion of the electrolytic capacitor, but the glycol compound hardlypasses through the sealed portion.

The proportion of the glycol compound contained in the solvent ispreferably 50% by mass or more, more preferably 60% by mass or more,still more preferably 70% by mass or more. When the electrolyticsolution contains the glycol compound as a main solvent, an effect ofreducing ESR of the electrolytic capacitor and an effect of improvingheat resistance are enhanced.

The glycol compound preferably contains at least ethylene glycol. Inaddition, when the solvent contains plural kinds of glycol compounds,ethylene glycol is preferably a main component among the glycolcompounds. Since ethylene glycol has low viscosity among glycolcompounds, ethylene glycol easily dissolves a solute. Further, sinceethylene glycol has high heat conductivity, ethylene glycol exhibitsexcellent heat dissipation when a ripple current has occurred. Thus,ethylene glycol has a large effect of improving the heat resistance.

The proportion of ethylene glycol in the glycol compound is preferably30% by mass or more, further preferably 50% by mass or more, further 70%by mass or more, and 100% by mass of the glycol compound may beconstituted by ethylene glycol.

The glycol compound may contain, for example, diethylene glycol,triethylene glycol, propylene glycol, polyethylene glycol having amolecular weight of from 190 to 400, inclusive, and the like in additionto ethylene glycol. For example, polyethylene glycol having a molecularweight of from 200 to 300, inclusive, may constitute 3% by mass or moreand 25% by mass or less of the solvent. By these configuration, heatresistance of the electrolytic capacitor can be further improved.

The acid component initially decreases pH (potential of hydrogen) of theelectrolytic solution, so that dedoping of a dopant from a conductivepolymer is suppressed. Dedoping of a dopant from the conductive polymeris considered to be one of causes for deterioration of the solidelectrolyte layer. The deterioration of the solid electrolyte layertends to increase the ESR, and decrease the breakdown voltage. In orderto suppress deterioration of the solid electrolyte layer, a mass of theacid component in a solute is need to be greater than a mass of the basecomponent in the solute. More specifically, a content ratio of the acidcomponent in the solute is required to exceed 100 parts by mass withrespect to 100 parts by mass of the base component, and preferably 130parts by mass or more.

However, even when the electrolytic solution includes the acid componentmore than the base component, it is difficult to stabilize theconductive polymer for a long period of time if the solvent contains aglycol compound. In this case, ESR tends to rapidly increase earlierthan expected.

With regard to this problem, by containing the first aromatic compoundhaving a hydroxyl group in the solute, it is possible to keep the ESR ofthe electrolytic capacitor low not only initially but also for a longperiod of time. The reason is considered to be because the hydroxylgroup of the first aromatic compound has an effect of stabilizing theconductive polymer for a long period of time. This effect is consideredto be related to the fact that the hydroxyl group of the first aromaticcompound exhibits mild acidity, and/or the fact that the hydroxyl groupof the first aromatic compound is less likely to cause a side reactionsuch as an esterification reaction to proceed because the hydroxyl groupof the first aromatic compound is stable.

The acid component may contain a carboxylic acid in addition to thefirst aromatic compound. However, when an insufficient amount ofcarboxylic acid is used in combination with the first aromatic compound,the effect of suppressing the increase in ESR due to the first aromaticcompound may be decreased in some cases. In addition, from the viewpointof improving the initial property and the long-term property whilesuppressing progress of the side reaction, the first aromatic compoundpreferably accounts for a significant proportion in the acid component.For example, the proportion of the first aromatic compound in the acidcomponent preferably ranges from 30% by mass to 100% by mass, inclusive,more preferably ranges from 36% by mass to 100% by mass, inclusive.

In order to suppress the increase in ESR when the electrolytic solutioncontains no first aromatic compound, a large amount of carboxylic acidneeds to be added in the electrolytic solution. In this case, acarboxylic acid that is at least twice or more as large in mass as thebase component is needed. In consideration of the suppression of theside reaction and the reduction of raw material cost, it is preferableto use the first aromatic compound in place of using a large amount ofcarboxylic acid in many aspects.

An aromatic ring of the first aromatic compound is preferably a C6benzene ring or a C10 naphthyl ring from the viewpoint of suppressing anincrease in viscosity of the electrolytic solution. In addition, thefirst aromatic compound preferably has one or more phenolic hydroxylgroups bonded directly to the aromatic ring for securing long-termstability. And the first aromatic compound preferably includes, forexample, phenol, dibutylhydroxyltoluene, cresol, methoxyphenol, eugenol,guaiacol, thymol, catechol, pyrogallol, or the like. Among thesecompounds, divalent to tetravalent phenolic compounds which have two tofour phenolic hydroxyl groups are preferably used as the first aromaticcompound. More specifically, it is more preferable to use at least oneselected from the group consisting of catechol and pyrogallol as thefirst aromatic compound. Further, pyrogallol is especially preferablebecause it is moderately acidic. And it is preferable that 90% by massor more of the first aromatic compound is constituted by pyrogallol.

Among aromatic compounds having a phenolic hydroxyl group, an aromaticcompound having a carboxyl group bonded directly to an aromatic ringexhibits relatively strong acidity due to the carboxyl group. Inparticular, an aromatic compound having a carboxyl group at anortho-position adjacent to the hydroxyl group (for example, salicylicacid) exhibits strong acidity. Although the reason is not clear, use ofsuch an aromatic compound fails to have any effect of stabilizing theconductive polymer over a period of time, and thus it is difficult tosuppress the increase in ESR for a long period of time. Therefore, thefirst aromatic compound is required to be an aromatic compound having nocarboxyl group bonded directly to an aromatic ring.

From the viewpoint of further improving heat resistance of theelectrolytic capacitor, and further suppressing deterioration of thesolid electrolyte layer, the proportion of the first aromatic compoundin the electrolytic solution preferably ranges from 0.1% by mass to 30%by mass, inclusive, more preferably from 2% by mass to 25% by mass,inclusive. In addition, from the same viewpoint, the proportion of thefirst aromatic compound in a total of the solute preferably ranges from20% by mass to 95% by mass, inclusive, more preferably from 30% by massto 90% by mass, inclusive.

The pH of the electrolytic solution is preferably 6 or less, morepreferably 4 or less, still more preferably 3.8 or less, or 3.6 or less.When the pH of the electrolytic solution is 4 or less, deterioration ofthe conductive polymer is further suppressed. Generally, it isconsidered that an anode body is corroded when the pH of an electrolyticsolution is 4 or less. However, when the above-mentioned electrolyticsolution is used, corrosion of the anode body is also suppressed.Further, the pH of the electrolytic solution is more preferably 2.0 ormore.

As the carboxylic acid that can be used in combination with the firstaromatic compound, an aromatic compound which has no phenolic hydroxylgroup, and has two or more carboxyl groups (second aromatic compound) ispreferable. The carboxyl group of the second aromatic compound isrelatively stable, so that a side reaction hardly proceeds. The secondaromatic compound exhibits an effect of stabilizing the conductivepolymer over a relatively long period of time. The second aromaticcompound is moderately acidic in the electrolytic solution, so that theanode body is less likely damaged by corrosion.

An aromatic ring of the second aromatic compound is preferably a C6benzene ring or a C10 naphthyl ring from the viewpoint of suppressing anincrease in viscosity of the electrolytic solution. In addition, thesecond aromatic compound is preferably a divalent to tetravalentcarboxylic acid because it is moderately acidic. More preferably, thesecond aromatic compound has at least two or more carboxyl groups bondeddirectly to an ortho-position of the aromatic ring because the carboxylgroup is easily stabilized. More specifically, it is more preferable touse at least one selected from the group consisting of o-phthalic acidand pyromellitic acid as the second aromatic compound. Further,o-phthalic acid is especially preferable because the carboxyl group iseasily stabilized, thereby exhibiting the effect of stabilizing theconductive polymer over a longer period of time. And it is preferablethat 90% by mass or more of the second aromatic compound is constitutedby o-phthalic acid.

A part of the carboxylic acid may be derived from a salt of thecarboxylic acid component and the base component. That is, a salt of thecarboxylic acid and the base component may be used as a part of thesolute. By using such a salt, an effect that a degree of dissociation ofthe carboxylic acid is improved can be obtained. For example, 10% bymass or more and 50% by mass or less of the carboxylic acid ispreferably derived from a salt of the carboxylic acid component and thebase component.

The base component is preferably at least one selected from the groupconsisting of a primary amine, a secondary amine, and a tertiary amine.By using an amine component, particularly primary to tertiary amine, aneffect of stabilizing ESR for a long period of time is enhanced.Although a quaternary amine may be used, from the viewpoint ofsuppressing a side reaction as much as possible, primary to tertiaryamines are preferable because they are moderately basic. As each of theamines, it is possible to use an aliphatic amine, an aromatic amine, anda heterocyclic amine. However, an aliphatic amine having a molecularweight ranging from 72 to 102, inclusive, is preferable because such analiphatic amine has a high degree of dissociation.

Examples of the primary to tertiary amine include methyl amine, dimethylamine, trimethyl amine, ethyl amine, diethyl amine, triethyl amine,ethylene diamine, N,N-diisopropylethyl amine, tetramethylethylenediamine, hexamethylene diamine, spermidine, sp ermine, amantadine,aniline, phenethylamine, toluidine, pyrrolidine, piperidine, piperazine, morpholine, imidazole, pyridine, pyridazine, pyrimidine,pyrazine, and 4-dimethylaminopyridine. These amines may be used alone,or two or more of the amines may be used in combination. Among theseamines, tertiary amines such as triethyl amine and monoethyldimethylamine are particularly preferable.

The proportion of the solute in the electrolyte solution preferablyranges 2% by mass to 32% by mass, inclusive, more preferably from 2% bymass to 10% by mass from the viewpoint of easily achieving the effect ofsuppressing deterioration of the solid electrolyte layer for a longperiod of time. For example, the proportion of a total of the firstaromatic compound, the base component, and the carboxylic acid (orsecond aromatic compound) in the electrolytic solution preferably rangesfrom 2% by mass to 32% by mass, inclusive, more preferably from 2% bymass to 10% by mass, inclusive.

The content ratio of the carboxylic acid preferably exceeds 200 parts bymass with respect to 100 parts by mass of the base component. However,an excess of carboxylic acid reduces the merit of using the firstaromatic compound. Therefore, the content ratio of the carboxylic acidis preferably 500 parts by mass or less with respect to 100 parts bymass of the base component.

In addition, the proportion of the carboxylic acid or the secondaromatic compound in the entire solute is preferably 60% by mass orless, more preferably 50% by mass or less. By this configuration, theside reaction is suppressed, and thus, for example, it is possible toreduce an amount of water increased in the electrolytic capacitor duringa long time use.

The solvent may contain, for example, a sulfone compound, a lactonecompound, a carbonate compound, and the like in addition to the glycolcompound. As the sulfone compound, sulfolane, dimethyl sulfoxide,diethyl sulfoxide, and the like can be used. As the lactone compound,γ-butyrolactone, γ-valerolactone, and the like can be used. As thecarbonate compound, dimethyl carbonate (DMC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylenecarbonate (PC), fluoroethylene carbonate (FEC), and the like can beused. These compounds may be used alone, or the multiple types ofcompounds may be used in combination.

The solid electrolyte layer may be formed by a method that chemicalpolymerization or electrolytic polymerization is performed on thedielectric layer by applying a solution containing, for example, amonomer, a dopant, and an oxidant to the dielectric layer. However, thesolid electrolyte layer is preferably formed by a method that theconductive polymer is applied to the dielectric layer from the reasonthat excellent withstand voltage characteristics can be expected. Morespecifically, the solid electrolyte layer is preferably formed byimpregnating the dielectric layer with a polymer dispersion containing aliquid component and the conductive polymer dispersed in the liquidcomponent (particularly, a polymer dispersion containing the conductivepolymer and the polymer dopant) so as to form a film that covers atleast a part of the dielectric layer, and then volatilizing the liquidcomponent from the film. The electrolytic solution described above isparticularly effective for suppressing deterioration of the conductivepolymer contained in the polymer dispersion, and is also effective forimproving the orientation of the conductive polymer.

A concentration of the conductive polymer in the polymer dispersion ispreferably from 0.5% by mass to 10% by mass, inclusive. For example, anaverage particle diameter D50 of the conductive polymer is preferablyfrom 0.01 μm to 0.5 μm, inclusive. Here, the average particle diameterD50 is a median diameter in a volume particle size distribution obtainedby a particle size distribution measuring apparatus according to dynamiclight scattering. The polymer dispersion having such a concentration issuitable for forming a solid electrolyte layer having an appropriatethickness and is easily impregnated into the dielectric layer.

The conductive polymer included in the solid electrolyte layer ispreferably, for example, polypyrrole, polythiophene, or polyaniline.These conductive polymers may be used alone, or two or more of theconductive polymers may be used in combination, or a copolymer of two ormore monomers may be adopted. The solid electrolyte layer including sucha conductive polymer can be expected to further improve the withstandvoltage characteristics.

In the present specification, polypyrrole, polythiophene, polyaniline,and the like mean polymers having, as a basic skeleton, polypyrrole,polythiophene, polyaniline, and the like, respectively. Therefore,polypyrrole, polythiophene, polyaniline, and the like also includederivatives of polypyrrole, polythiophene, polyaniline, and the like,respectively. For example, polythiophene includespoly(3,4-ethylenedioxythiophene) (PEDOT) and the like.

The weight average molecular weight of the conductive polymer is notparticularly limited and ranges, for example, from 1000 to 100000,inclusive.

From the viewpoint of suppressing the dedoping of a dopant from theconductive polymer, the solid electrolyte layer preferably includes apolymer dopant. Examples of the polymer dopant include a polyanion of,for example, polyvinylsulfonic acid, polystyrenesulfonic acid,polyallylsulfonic acid, polyacrylsulfonic acid, polymethacrylsulfonicacid, poly(2-acrylamido-2-methylpropanesulfonic acid),polyisoprenesulfonic acid, and polyacrylic acid. These polyanions may beused alone, or two or more of the polyanions may be used in combination.These polymer dopants may be a homopolymer or a copolymer of two or moremonomers. Especially, polystyrenesulfonic acid (PSS) is preferable.

The weight average molecular weight of the polymer dopant is notparticularly limited and preferably ranges, for example, from 1000 to100000, inclusive, from the viewpoint of facilitating formation of ahomogeneous solid electrolyte layer.

Hereinafter, the present disclosure is more specifically described withreference to the exemplary embodiment. The exemplary embodimentdescribed below, however, is not to be considered to limit the presentdisclosure.

FIG. 1 is a schematic sectional view illustrating the electrolyticcapacitor according to the present exemplary embodiment, and FIG. 2 is aschematic view obtained by developing a part of a capacitor elementaccording to the electrolytic capacitor.

The electrolytic capacitor includes, for example, capacitor element 10,bottomed case 11 that houses capacitor element 10, sealing member 12that seals an opening of bottomed case 11, base plate 13 that coverssealing member 12, lead wires 14A, 14B that are lead out from sealingmember 12 and penetrate base plate 13, lead tabs 15A, 15B that connectthe lead wires to electrodes of capacitor element 10, respectively, andan electrolytic solution (not shown). Bottomed case 11 is, at a partnear an opening end, processed inward by drawing, and is, at the openingend, curled to swage sealing member 12.

Sealing member 12 is formed of an elastic material containing a rubbercomponent. As the rubber component, there can be used a butyl rubber(IIR), a nitrile rubber (NBR), an ethylene propylene rubber, an ethylenepropylene diene rubber (EPDM), a chloroprene rubber (CR), an isoprenerubber (IR), a Hypalon (trademark) rubber, a silicone rubber, and afluorine-containing rubber. Sealing member 12 may contain fillers suchas carbon black and silica.

In design of the electrolytic solution, it is necessary to considervolatilization of the electrolytic solution to outside through sealingmember 12 that forms a sealed portion. In this respect, the electrolyticsolution according to the present exemplary embodiment contains theglycol compound, and therefore hardly passes through the sealed portioneven at a high temperature. Accordingly, an electrolytic capacitorhaving excellent heat resistance is obtained.

Capacitor element 10 is formed of a wound body as illustrated in FIG. 2.The wound body is a semi-manufactured product of capacitor element 10and refers to a capacitor element in which a solid electrolyte layer hasnot yet been formed between anode body 21 on a surface of which adielectric layer is provided and cathode body 22. The wound bodyincludes anode body 21 connected to lead tab 15A, cathode body 22connected to lead tab 15B, and separator 23.

Anode body 21 and cathode body 22 are wound with separator 23 interposedbetween anode body 21 and cathode body 22. An outermost periphery of thewound body is fixed with fastening tape 24. FIG. 2 shows partiallydeveloped wound body before the outermost periphery of the wound body isfixed.

Anode body 21 includes a metal foil whose surface is roughened so as tohave projections and recesses, and the dielectric layer is formed on themetal foil having the projections and recesses. The conductive polymeris attached to at least a part of a surface of the dielectric layer toform the solid electrolyte layer. The solid electrolyte layer may coverat least a part of a surface of cathode body 22 and/or at least a partof a surface of separator 23. Capacitor element 10 in which the solidelectrolyte layer has been formed is housed in an outer case togetherwith the electrolytic solution.

Method for Producing Electrolytic Capacitor

Hereinafter, described are steps of one exemplary method for producingthe electrolytic capacitor according to the present exemplaryembodiment.

(i) Step of Preparing Anode Body 21 with Dielectric Layer

First, a metal foil as a raw material for anode body 21 is prepared. Atype of the metal is not particularly limited, but it is preferable touse a valve metal such as aluminum, tantalum, or niobium, or an alloyincluding a valve metal, from the viewpoint of facilitating formation ofthe dielectric layer.

Next, a surface of the metal foil is roughened. By the roughening, aplurality of projections and recesses are formed on the surface of themetal foil. The roughening is preferably performed by etching the metalfoil. The etching may be performed by, for example, a direct-currentelectrolytic method or an alternating-current electrolytic method.

Next, a dielectric layer is formed on the roughened surface of the metalfoil. A method for forming the dielectric layer is not particularlylimited, and the dielectric layer can be formed by subjecting the metalfoil to an anodizing treatment. The anodizing treatment is performed by,for example, immersing the metal foil in an anodizing solution such asan ammonium adipate solution followed by a heat treatment. The anodizingtreatment may also be performed by applying a voltage to the metal foilthat has been immersed in the anodizing solution.

Normally, a large foil of, for example, a valve metal (metal foil) issubjected to the roughening treatment and the anodizing treatment fromthe viewpoint of mass productivity. In this case, the treated foil iscut into a desired size to prepare anode body 21.

(ii) Step of Preparing Cathode Body 22

A metal foil can be used for cathode body 22 as with the anode body. Atype of the metal is not particularly limited, but it is preferred touse a valve metal such as aluminum, tantalum, or niobium, or an alloyincluding a valve metal. A surface of cathode body 22 may be roughenedas necessary.

(iii) Manufacturing of Wound Body

Next, anode body 21 and cathode body 22 are used to manufacture a woundbody.

First, anode body 21 and cathode body 22 are wound with separator 23interposed between anode body 21 and cathode body 22. At this time, thewinding can be conducted while lead tabs 15A, 15B are rolled in anodebody 21, cathode body 22, and separator 23, to cause lead tabs 15A, 15Bto stand up from the wound body as illustrated in FIG. 2.

As a material for separator 23, a nonwoven fabric can be used thatincludes, as a main component, for example, synthetic cellulose,polyethylene terephthalate, a vinylon, or an aramid fiber.

A material for lead tabs 15A, 15B is not also particularly limited aslong as the material is a conductive material. A material for lead wires14A, 14B connected to lead tabs 15A, 15B, respectively, is not alsoparticularly limited as long as the material is a conductive material.

Next, fastening tape 24 is disposed on an outer surface of cathode body22 positioned at an outermost layer of wound anode body 21, cathode body22, and separator 23, to fix an end of cathode body 22 with fasteningtape 24. When anode body 21 is prepared by cutting a large metal foil,the wound body may further be subjected to an anodizing treatment inorder to provide a dielectric layer on a cutting surface of anode body21.

(iv) Step of Forming Capacitor Element 10

Next, the dielectric layer is impregnated with a polymer dispersion toform a film covering at least a part of the dielectric layer. Thepolymer dispersion contains a liquid component and a conductive polymerdispersed in the liquid component. The polymer dispersion may be asolution obtained by dissolving the conductive polymer in the liquidcomponent, or a dispersion liquid obtained by dispersing particles ofthe conductive polymer in the liquid component. Next, the formed film isdried to volatilize the liquid component from the film, forming a densesolid electrolyte layer covering at least a part of the dielectriclayer. The polymer dispersion is uniformly distributed in the liquidcomponent to easily form a uniform solid electrolyte layer. Thus,capacitor element 10 can be obtained.

The polymer dispersion can be obtained by, for example, a method fordispersing the conductive polymer in the liquid component. Or thepolymer dispersion can also be obtained by a method for polymerizing aprecursor monomer in the liquid component and generating particles ofthe conductive polymer. Preferable examples of the polymer dispersioninclude, for example, poly(3,4-ethylenedioxythiophene) (PEDOT) dopedwith polystyrenesulfonic acid (PSS), i.e., PEDOT/PSS. An antioxidant forthe conductive polymer may be added. However, it is unnecessary to usean antioxidant because PEDOT/PSS little oxidizes.

The liquid component may be water, a mixture of water and a nonaqueoussolvent, or a nonaqueous solvent. The nonaqueous solvent is notparticularly limited, and a protic solvent and an aprotic solvent can beused, for example. Examples of the protic solvent include alcohols suchas methanol, ethanol, propanol, butanol, ethylene glycol, and propyleneglycol, formaldehyde, and ethers such as 1,4-dioxane. Examples of theaprotic solvent include amides such as N-methylacetamide,N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methylacetate, and ketones such as methyl ethyl ketone.

The method for applying the polymer dispersion to a surface of thedielectric layer is preferably a method for immersing the wound body inthe polymer dispersion housed in a container because the method issimple. An immersion period depends on a size of the wound body, butranges, for example, from 1 second to 5 hours, inclusive, preferablyfrom 1 minute to 30 minutes, inclusive. In addition, impregnation ispreferably performed under a reduced pressure, in a pressure of anatmosphere ranging, for example, from 10 kPa to 100 kPa, inclusive,preferably from 40 kPa to 100 kPa, inclusive. Further, ultrasonicvibration may be applied to the wound body or the polymer dispersionwhile the wound body is immersed in the polymer dispersion. The dryingafter picking the wound body up from the polymer dispersion is performedat a temperature ranging preferably from 50° C. to 300° C., inclusive,more preferably 100° C. to 200° C., inclusive, for example.

The step of applying the polymer dispersion to the surface of thedielectric layer and the step of drying the wound body may be repeatedtwo or more times. These steps can be performed a plurality of times toincrease coverage of the solid electrolyte layer on the dielectriclayer. In the steps, the solid electrolyte layer may be formed on notonly the surface of the dielectric layer but also surfaces of cathodebody 22 and separator 23.

As described above, the solid electrolyte layer is formed between anodebody 21 and cathode body 22 to manufacture capacitor element 10. Thesolid electrolyte layer formed on the surface of the dielectric layervirtually functions as a cathode material.

(v) Step of Impregnating Capacitor Element 10 with Electrolytic Solution

Next, capacitor element 10 is impregnated with an electrolytic solution.This provides an electrolytic capacitor excellent in a repairingfunction of a dielectric layer. A method for impregnating capacitorelement 10 with an electrolytic solution is not particularly limited.For example, a method for immersing capacitor element 10 in theelectrolytic solution housed in a container is simple and preferred. Animmersion period depends on a size of capacitor element 10, and ranges,for example, from 1 second to 5 minutes, inclusive. Impregnation ispreferably performed under a reduced pressure, in a pressure of anatmosphere ranging, for example, from 10 kPa to 100 kPa, inclusive,preferably from 40 kPa to 100 kPa, inclusive.

(iv) Step of Encapsulating Capacitor Element

Next, capacitor element 10 is encapsulated. Specifically, first,capacitor element 10 is housed in bottomed case 11 so that lead wires14A, 14B are positioned on an open upper surface of bottomed case 11. Asa material for bottomed case 11, there can be used metals such asaluminum, stainless steel, copper, iron and brass, or alloys of thesemetals.

Next, sealing member 12 formed so as to allow lead wires 14A, 14B topenetrate the sealing member is disposed above capacitor element 10 toencapsulate capacitor element 10 in bottomed case 11. Next, bottomedcase 11 is, at a part near an opening end, processed by transversedrawing, and is, at the opening end, curled to swage sealing member 12.Then, base plate 13 is disposed on a curled part of the bottomed case tocomplete the electrolytic capacitor as illustrated in FIG. 1. Then, anaging treatment may be performed while a rated voltage is applied.

In the exemplary embodiment described above, a wound electrolyticcapacitor has been described. The application range of the presentdisclosure, however, is not limited to the wound electrolytic capacitorand can also be applied to other electrolytic capacitors such as a chipelectrolytic capacitor including a metal sintered body as an anode body,and a laminated electrolytic capacitor including a metal plate as ananode body.

EXAMPLES

Hereinafter, the present disclosure is described in more detail withreference to examples. The present disclosure, however, is not to beconsidered limited to the examples.

Examples 1 to 6

In the present example, a wound electrolytic capacitor (Φ: (diameter)8.0 mm×L (length): 12.0 mm) having a rated voltage of 100 V and a ratedelectrostatic capacity of 15 μF was produced. Hereinafter, a specificmethod for producing the electrolytic capacitor is described.

Preparation of Anode Body

A 100-μm-thick aluminum foil was subjected to etching to roughen asurface of the aluminum foil. Then, a dielectric layer was formed on thesurface of the aluminum foil by an anodizing treatment. The anodizingtreatment was performed by immersing the aluminum foil in an ammoniumadipate solution and applying a voltage of 60 V to the aluminum foil.Then, the aluminum foil was cut into a size of 6 mm (length)×120 mm(width) to prepare an anode body.

Preparation of Cathode Body

A 50-μm-thick aluminum foil was subjected to etching to roughen asurface of the aluminum foil. Then, the aluminum foil was cut into asize of 6 mm (length)×120 mm (width) to prepare a cathode body.

Manufacturing of Wound Body

An anode lead tab and a cathode lead tab were connected to the anodebody and the cathode body, respectively, and the anode body and thecathode body were wound with a separator interposed between the anodebody and the cathode body while the lead tabs were rolled in the anodebody, the cathode body, and the separator. Ends of the lead tabsprotruding from the wound body were connected to an anode lead wire anda cathode lead wire, respectively. Then, the manufactured wound body wassubjected to an anodizing treatment again to form a dielectric layer ata cutting end of the anode body. Next, an end of an outer surface of thewound body was fixed with a fastening tape to complete the wound body.

Preparation of Polymer Dispersion

A mixed solution was prepared by dissolving 3,4-ethylenedioxythiopheneand a polymer dopant, i.e., polystyrenesulfonic acid (PSS, weightaverage molecular weight 100000) in ion-exchanged water (liquidcomponent). While the mixed solution was stirred, iron (III) sulfate(oxidant) that had been dissolved in ion-exchanged water was added tothe mixed solution to cause a polymerization reaction. After thereaction, a resultant reaction solution was dialyzed to remove unreactedmonomers and an excessive oxidant, so that a polymer dispersion wasobtained that contained about 5% by mass of polyethylene dioxythiophenedoped with PSS (PEDOT/PSS).

Formation of Solid Electrolyte Layer

The wound body was immersed in the polymer dispersion housed in apredetermined container in a reduced-pressure atmosphere (40 kPa) for 5minutes, and then the wound body was picked up from the polymerdispersion. Next, the wound body that had been impregnated with thepolymer dispersion was dried in a drying furnace at 150° C. for 20minutes to form a solid electrolyte layer covering at least a part ofthe dielectric layer.

Impregnation with Electrolytic Solution

An electrolytic solution containing ethylene glycol (EG) andpolyethylene glycol (PEG) with a molecular weight of about 300 as glycolcompounds and containing pyrogallol as the first aromatic compound toprovide a composition as shown in Table 1 below was prepared. And acapacitor element was immersed in the electrolytic solution in areduced-pressure atmosphere (40 kPa) for 5 minutes. A salt of thecarboxylic acid (o-phthalic acid) and the base component (triethylamine), which is triethylamine phthalate, was added as at least a partof the carboxylic acid.

TABLE 1 Electrolytic capacitor A1 A2 A3 A4 A5 A6 Proportions ofrespective components in solvent EG (% by mass) 60 60 60 60 60 60 PEG (%by mass) 20 20 20 20 20 20 Glycerin (% by mass) 20 20 20 20 20 20 Totalof solvent (% by mass) 100 100 100 100 100 100 Proportion of glycolcompound 80 80 80 80 80 80 in solvent (% by mass) Proportion of EG 75 7575 75 75 75 in glycol compound (% by mass) Proportion of PEG 25 25 25 2525 25 in glycol compound (% by mass) Ratio of solute component vs 100parts by mass of solvent Triethylamine (parts by mass) 2.2 2.2 1.0 2.20.05 2.2 o-phthalic acid (parts by mass) 0 0 0 0 0 5.2 Pyrogallol (partsby mass) 3 45 15 10 0.1 3 Total of solute (parts by mass) 5.2 47.2 1612.2 0.15 10.4 Proportion of base component 42.3 4.7 6.3 18 33.3 21.2 insolute (% by mass) Proportion of pyrogallol 57.7 95.3 93.7 82.0 66.728.8 in solute (% by mass) Proportion of o-phthalic acid 0 0 0 0 0 50 insolute (% by mass) Ratio of acid component 136 2046 1500 455 200 373with respect to 100 parts by mass of base component (parts by mass) pHof electrolytic solution 6.1 3.4 3.7 3.8 6.6 4.3 Proportion ofpyrogallol 2.9 30.6 12.9 8.9 0.1 2.7 in electrolytic solution (% bymass) Proportion of solute 4.9 32.1 13.8 10.9 0.1 9.4 in electrolyticsolution (% by mass)

Encapsulation of Capacitor Element

The capacitor element that had been impregnated with the electrolyticsolution was encapsulated to complete an electrolytic capacitor.Specifically, the capacitor element was housed in a bottomed case sothat lead wires were positioned on an opening side of the bottomed case.And then a sealing member (an elastic material including a butyl rubberas a rubber component) that was formed so as to allow the lead wires topenetrate the sealing member was disposed above the capacitor element,so that encapsulate the capacitor element was encapsulated in thebottomed case. The bottomed case was, at a part near an opening end,processed by drawing and was further curled at the opening end, and abase plate was disposed on a curled part to complete electrolyticcapacitors (A1 to A6) as shown in FIG. 1. Thereafter, an aging treatmentwas performed at 130° C. for 2 hours while a rated voltage was applied.The electrolytic capacitors A1 to A6 respectively correspond to Examples1 to 6.

Comparative Example 1

Except for using no pyrogallol, and for using an electrolytic solutionof composition as shown in Table 2, where the ratio of o-phthalic acidto 100 parts by mass of the solvent was changed to 4.2 parts by mass, anelectrolytic capacitor B1 was prepared in the same way as in Example 6.

Comparative Example 2

Except for changing the ratio of pyrogallol to 100 parts by mass of thesolvent to 0.5 parts by mass, and for using an electrolytic solution ofcomposition as shown in Table 2, where the ratio of o-phthalic acid waschanged to 1.6 parts by mass, an electrolytic capacitor B2 was preparedin the same way as in Example 6.

TABLE 2 Electrolytic capacitor B1 B2 Proportions of respectivecomponents in solvent EG (% by mass) 60 60 PEG (% by mass) 20 20Glycerin (% by mass) 20 20 Total of solvent (% by mass) 100 100Proportion of glycol compound 80 80 in solvent (% by mass) Proportion ofEG 75 75 in glycol compound (% by mass) Proportion of PEG 25 25 inglycol compound (% by mass) Ratio of solute component vs 100 parts bymass of solvent Triethylamine (parts by mass) 2.2 2.2 o-phthalic acid(parts by mass) 4.2 1.6 Pyrogallol (parts by mass) 0 0.5 Total of solute(parts by mass) 6.4 4.3 Proportion of base component 34.4 51.2 in solute(% by mass) Proportion of pyrogallol 0 11.6 in solute (% by mass)Proportion of o-phthalic acid 65.6 37.2 in solute (% by mass) Ratio ofacid component 191 95.5 with respect to 100 parts by mass of basecomponent (parts by mass) pH of electrolytic solution 4.7 7.1 Proportionof pyrogallol 0 0.5 in electrolytic solution (% by mass) Proportion ofsolute 6.0 4.1 in electrolytic solution (% by mass)

Examples 7 to 11

Electrolytic capacitors A7 to A11 were manufactured in the same manneras in Example 1 except that the composition of the solvent were changedas shown in Table 3, and the evaluation was performed in the samemanner.

TABLE 3 Electrolytic capacitor A7 A8 A9 A10 A11 Proportions ofrespective components in solvent EG (% by mass) 80 50 40 30 20 PEG (% bymass) 20 25 10 10 20 Glycerin (% by mass) 0 25 50 60 60 Total of solvent(% by mass) 100 100 100 100 100 Proportion of glycol compound 100 75 5040 40 in solvent (% by mass) Proportion of EG 80 67 80 75 50 in glycolcompound (% by mass) Proportion of PEG 20 33 20 25 50 in glycol compound(% by mass) Ratio of solute component vs 100 parts by mass of solventTriethylamine (parts by mass) 2.2 2.2 2.2 2.2 2.2 o-phthalic acid (partsby mass) 0 0 0 0 0 Pyrogallol (parts by mass) 3 3 3 3 3 Total of solute(parts by mass) 5.2 5.2 5.2 5.2 5.2 Proportion of base component 42.342.3 42.3 42.3 42.3 in solute (% by mass) Proportion of pyrogallol 57.757.7 57.7 57.7 57.7 in solute (% by mass) Proportion of o-phthalic acid0 0 0 0 0 in solute (% by mass) Ratio of acid component 136 136 136 136136 with respect to 100 parts by mass of base component (parts by mass)pH of electrolytic solution 6.2 6.1 6.1 6.2 6.1 Proportion of pyrogallol2.9 2.9 2.9 2.9 2.9 in electrolytic solution (% by mass) Proportion ofsolute 4.9 4.9 4.9 4.9 4.9 in electrolytic solution (% by mass)

Evaluation

Electrostatic capacity, ESR, and a breakdown voltage (BDV) were measuredfor the resultant electrolytic capacitors. The breakdown voltage (BDV)was defined as a voltage measured when the voltage was applied at anincreasing rate of 1.0 V/s and an excess current of 0.5 A flowed.

Further, in order to evaluate long term reliability, the electrolyticcapacitor was retained at 125° C. for 5000 hours while a rated voltagewas applied, and an increase rate in ESR (ΔESR) was evaluated. Theincrease rate ΔESR was represented by a ratio (X/X₀) of a value of ESR(X) after retention for 5000 hours to an initial value (X₀). Table 4shows evaluation results.

TABLE 4 Electrostatic Electrolytic capacity ESR BDV capacitor (μF) (Ω)(V) X/X₀ A1 32.1 15.0 145.0 1.7 A2 32.0 16.7 144.0 1.5 A3 32.0 16.6144.0 1.5 A4 32.2 16.5 146.0 1.5 A5 32.0 17.1 144.0 2.7 A6 32.1 16.0149.0 1.6 A7 32.5 15.0 145.0 1.7 A8 31.8 16.8 146.0 1.9 A9 31.8 17.2146.0 1.8 A10 31.6 17.1 147.0 1.7 A11 31.5 17.3 146.0 1.8 B1 31.9 15.8149.0 266.0 B2 32.1 16.5 148.0 72.6

The present disclosure can be utilized for an electrolytic capacitorthat includes a solid electrolyte layer covering at least a part of adielectric layer, and an electrolytic solution.

What is claimed is:
 1. An electrolytic capacitor comprising: an anodebody having a dielectric layer; a solid electrolyte layer in contactwith the dielectric layer of the anode body; and an electrolyticsolution, wherein: the electrolytic solution contains a solvent and asolute, the solvent contains a glycol compound, the solute contains anacid component, the acid component contains a carboxylic acid and afirst aromatic compound, the first aromatic compound having a hydroxylgroup and no carboxyl group bonded directly to an aromatic ring of thefirst aromatic compound, a proportion of the first aromatic compound inthe acid component is more than or equal to 30% by mass, and aproportion of the glycol compound in the solvent is 50% by mass or more.2. The electrolytic capacitor according to claim 1, wherein the glycolcompound contains at least one selected from the group consistingethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, and polyethylene glycol having a molecular weight of from 190 to400, inclusive.
 3. The electrolytic capacitor according to claim 1,wherein the glycol compound contains ethylene glycol.
 4. Theelectrolytic capacitor according to claim 1, wherein the electrolyticsolution has a pH of 4 or less.
 5. The electrolytic capacitor accordingto claim 1, wherein the first aromatic compound has two or more hydroxylgroups.
 6. The electrolytic capacitor according to claim 5, wherein thefirst aromatic compound is at least one selected from a group consistingof catechol and pyrogallol.
 7. The electrolytic capacitor according toclaim 1, wherein a proportion of the first aromatic compound in theelectrolytic solution ranges from 0.1% by mass to 30% by mass,inclusive.
 8. The electrolytic capacitor according to claim 1, wherein aproportion of the solute in the electrolytic solution ranges from 2% bymass to 32% by mass, inclusive.
 9. The electrolytic capacitor accordingto claim 1, wherein a part of the carboxylic acid is derived from a saltof the carboxylic acid and a base component.
 10. The electrolyticcapacitor according to claim 1, wherein the carboxylic acid contains asecond aromatic compound having two or more carboxyl groups.
 11. Theelectrolytic capacitor according to claim 10, wherein the secondaromatic compound is at least one selected from a group consisting ofo-phthalic acid and pyromellitic acid.
 12. The electrolytic capacitoraccording to claim 9, wherein the base component is at least oneselected from a group consisting of a primary amine, a secondary amine,and a tertiary amine.
 13. The electrolytic capacitor according to claim1, wherein the solid electrolyte layer contains a conductive polymer anda polymer dopant.
 14. The electrolytic capacitor according to claim 1,wherein the first aromatic compound has at least one of a catecholskeleton and a pyrogallol skeleton.
 15. The electrolytic capacitoraccording to claim 1, wherein a proportion of the first aromaticcompound in the electrolytic solution ranges from 8.9% by mass to 30% bymass, inclusive.